Project Report PR-96-09
A study of the impact of Hurricane Iniki on coral communities at selected sites in Mamala Bay, Oahu, Hawaii
Richard E. Brock
Mamala Bay on Oahuºs south shore is the receiving body for sewage effluent discharged from two deep-ocean outfalls, the Sand Island Ocean Outfall and Barbers Point Ocean Outfall. The effluent, after receiving primary treatment at the Sand Island and Honouliuli wastewater treatment plants, is discharged about 2.7 km from shore at depths of 61 m and 71 m, respectively. The coral reef communities inshore of these outfalls are monitored at a number of permanently marked stations that take advantage of any shoreward gradient of stress that may emanate from these two point source discharges. Hurricane Iniki, which occurred in 1992, impacted coral reef communities in Mamala Bay and elsewhere. The damage caused by this storm could serve to obscure impacts due to the operation of the outfalls, and/or the decline in coral coverage could be construed as being the result of the operation of the outfalls. This study was undertaken to address these concerns. The study results revealed that pre-Iniki coral communities in Mamala Bay were not well-developed due to the occasional impact of storm-generated surf. Hurricane Iniki contributed to a decrease in coral coverage to varying degrees, with less impact in areas where local submarine topography served to protect corals from storm damage. Despite varying degrees of local impact, recovery in Mamala Bay coral communities was similar, irrespective of location or method of measurement. Recovery was measured by recording changes in coral coverage in permanently marked quadrats as well as changes in growth rates of marked and transplanted corals. The lack of significant differences in any of the growth parameters measured in this study between sites suggests that environmental parameters that may influence coral growth probably do not differ much between these locations. The recovery of corals at all sites following the hurricane was statistically similar, suggesting no negative influence from the operation of the deep-ocean outfalls. The greatest change attributable to the hurricane at the sites examined in this study was the loss of topographical relief and shelter habitat for many fish and invertebrate species. This loss was caused by the movement of loose materials (rubble and sand) across the bottom; their settling into depressions resulted in a less heterogeneous habitat. However, loss of habitat due to infilling is only one aspect of a continuum; major storm events may create topographical complexity and shelter space by uncovering previously buried substratum.